The custom design of a metal-supported catalyst is often determinative of process activity and selectivity in a reaction—crucial in almost every industrial process. Even minor adjustments to a catalyst or to catalyst synthesis conditions can drastically alter the catalytic properties, significantly impacting process activity and selectivity in a reaction.
In many reactions, metal-supported catalysts need to have the highest possible catalytically active metal surface area, or dispersion of the metal. This can be accomplished by depositing metal nano-sized particles on an inert support, which may be distributed in various configurations. Three main configurations include eggshell, egg yolk, and uniform distributions as shown in FIG. 1.
In catalysts with an eggshell distribution, the metal nanoparticles are concentrated at the exterior edge of the support. These eggshell catalysts are particularly desirable for reactions that are heat or mass transfer limited. An eggshell catalyst allows for a reaction to occur quickly on the surface of the catalyst without the pores of the catalyst becoming blocked. With an eggshell catalyst, the diffusion path length necessary for the reactants and products to travel is minimized, making this configuration best for fast reactions with strong diffusional restrictions, producing higher selectivity in reactions where diffusion is the limiting step. However, there has been limited research done in the field of eggshell catalysts due to their complexity and difficult synthetic approaches.
While the use of small metallic particles is very desirable in a catalyst, the particles have a tendency towards agglomeration. Agglomeration occurs when nanoparticles clump together, bonding with other metallic nanoparticles to form large clusters, typically around 20 to 100 nm. Due to the high expense of most metals as well as dispersion and surface area considerations as mentioned above, agglomeration typically renders synthesis of 2 nm particles on a catalyst support a very difficult process.
Currently in the art, various methods have been employed to synthesize a metal nanoparticle catalyst. One such method consists of using capping agents to prevent the metallic nanoparticles from bonding with one another. Capping agents bind with the catalyst to control the structural characteristics and prevent agglomeration of the particles. Capping agents however, act as a physical barrier to the reactants, restricting access to the catalyst and therefore must be removed before the actual reaction can begin. This creates a very short window of opportunity in which the reaction must take place, while the catalyst is active but before the particles agglomerate.
Other methods used to synthesize metallic nanoparticle catalysts are surfactants, calcination, chemical vapor techniques, carbon monoxide, custom supports, manipulation of pH levels, manipulation of pressure conditions, and long preparation times. Many of these reactions can require difficult techniques and can be very costly, particularly on a large scale. In addition, most of these metallic nanoparticle catalysts are not synthesized in an eggshell formation, as there has been little research done in this field to-date and the current synthesis is quite difficult. Most catalysts are produced with a uniform configuration, with a few in egg yolk configurations, and fewer still in the desirable eggshell configuration.
Catalysts are often used in hydrogenation reactions. There has been a long-felt but unsolved need in the chemical industry for an easier, quicker, and more affordable alternative to produce desirable catalyst compositions without the requisite laborious synthesis to be used in selective hydrogenation reactions. Even small improvements in reducing production costs can generate substantial savings. By cutting out wasteful steps and extra reaction requirements, the savings of time and money could be exponential.
The synthesis of 2-ethylhexanal via an aldol condensation reaction exemplifies a compound that is selectively hydrogenated without removing the aldehyde functionality. 2-Ethylhexanal is an important industrial chemical finding uses in a variety of applications such as perfumes, synthetic precursors for plasticizers, and 2-ethylhexanoic acid. The synthesis of 2-ethylhexanal disclosed in the art typically requires several synthetic steps, high pressures, multiple catalysts, and/or complex catalysts.
Using a palladium on titania eggshell type catalyst eliminates the need for a homogeneous catalyst to promote the aldol reaction to make 2-ethylhexenal. Typically, the aldol reaction is base catalyzed, using a caustic reagent such as sodium hydroxide. Although effective, this type of base catalysis can be problematic as it requires special handling of the base, downstream separation of the homogeneous base, and higher capital costs. Typically in these applications, the caustic catalyst is not separated but requires additional treatments of the waste stream to meet regulatory requirements. Furthermore, this synthetic approach increases costs as the catalyst is not recovered but wasted downstream.
The present invention would allow for a facile synthesis of a palladium eggshell catalyst using easily-controllable, various sized nanoparticles in a desirable eggshell formation. This catalyst synthetic scheme allows for a one-step reaction, occurring at atmospheric pressure, without the manipulation of reaction pH, the use of surfactants, capping agents, carbon monoxide, or chemical vapor deposition. The preparation time is relatively short and a custom support is not required. By varying only the reducing agent used in the reaction, eggshell distributed metal catalysts can be synthesized with varying nanoparticle sizes.
This inventive palladium on titania eggshell catalyst is able to use n-butyraldehyde as a feed towards the single-step synthesis of 2-ethylhexanal. Using this palladium on titania eggshell catalyst to convert n-butyraldehyde to 2-ethylhexanal provides several benefits not disclosed together in the art such as: 1) a single-step synthetic scheme; 2) the use of a single regenerable heterogeneous palladium on titania eggshell catalyst; 3) a synthesis performed at atmospheric pressure; and 4) a reaction providing a cleaner product separation downstream due to the lack of catalyst separation or disposal.